The important physical properties of the window for the energy and day-lighting performance are the thermal transmittance, the total solar energy transmittance and the light transmittance. These parameters are defined in European and international standards and are usually known as the U — value, g-value, and xv-value, respectively [9,10]. The U-value is expressed in the unit W/m2K and is thus equal to the thermal conductivity of the window. The g value, also called solar factor or solar heat gain coefficient, is given as the fraction of incident solar power entering through the window. It includes radiation absorbed by the window and then reemitted to the inside of the room. It is a dimensionless unit with a value between 0 and 1, and is frequently expressed in percent. The light transmittance is the fraction of visible light directly transmitted through the window. In international literature the U-value is usually quoted for the complete window, including frame and edge. However, in this paper the given U — and g-values are for the center of the glass. The g — and Tv-values are quoted for the glazed area and only for normal incidence. A detailed characterization of the window performance requires that these properties are also known for oblique angles of incidence of solar radiation [11]. For the appearance of the window also the light reflectance and colour rendering are of importance, but they do not usually enter energy calculations. Individual panes are also characterized by the thermal emissivity of the coated surface. The hemispherical emissivity is calculated from the normal emissivity which is given by the equation s=1-R, where R is the near normal infrared reflectance. The emissivity of uncoated glass is 0.84 while low — emissivity coatings can have values as low as 0.03.

Low-emissivity coatings for solar and thermal control of modern windows have led to a revolution in window technology. With emissivity values below 0.10 W/m2K windows with U-values well below 1 W/m2K can readily be manufactured and still have high light transmittance. This can be compared to a standard double glazed window with a U-value of around 2.7-3.0 W/m2K. Solar control coatings with double silver layers can have light transmittance which is a factor of two

higher than its solar transmittance. This is almost identical to the maximum possible limit. In general we talk about low-e windows for cold climates and solar control windows for hot climates. The low-e window is characterised by a low U-value and a high g-value. Solar control windows tend to have low g-values and may or may not have low U-values, depending on the window type. Some solar control glazing products are based on absorption in the glass itself and not on a coating. Some solar control coatings are low-emissivity coatings and some are not. Low-e solar control coatings are usually based on one or two thin silver films and are also known as solar selective coatings. They are highly reflective in the near infrared part of the solar spectrum.